More than 40 states now have some sort of restriction on the use of tanning salons by minors, according to AIM at Melanoma, an advocacy and research group based in California, the first state to adopt a ban on minors in 2011.

There were about 14,000 salons across the country as of early 2014, according to John Overstreet, executive director of the Indoor Tanning Association.

The problem with indoor tanning, researchers say, is that many of those who do it, do it a lot. The federal government has collected data on tanning among high school students only since 2009, but researchers were surprised at the findings: Among those who used tanning beds, more than half had used them 10 times or more in the past year, according to Dr. Guy.

A panel of experts convened by the World Health Organization found in 2009 that the use of sun beds before age 30 was associated with a 75 percent increase in the risk of melanoma. A 2012 study found a 15 percent increase in the risk of certain skin cancers with every four sessions in a tanning bed before age 35.